1. Field of the Invention
The present invention relates to novel water-swellable fibers having a multiple layer structure consisting of an outer layer of a hydrophilic cross-linked polymer (hereinafter abbreviated as hydrogel) and an inner layer of an acrylonitrile polymer (hereinafter abbreviated as AN polymer) and/or another polymer, and to a process for producing the same.
2. Description of the Prior Art
In recent years, polymers having a high degree of water-swellability are used in a wide field of applications because of their particular functions. For example, attempts are made on applications to drapers, sanitary products, etc. by utilizing the instantaneous high water-absorbing power of these polymers; or applications to soil-improving materials, instant sandbags, etc. by utilizing their high water-retentive capacity; or applications to soft contact lenses, artificial internal organs, surgical seaming materials, etc. on account of their intimate affinity to human tissues, and among these applications some are already entering on a practical stage.
As regards the water-swellable polymers (hydrogels) having possibilities of use in such a wide field of applications, cases are not few where it is preferable that they take the form of fibers to meet their use purposes, and several hydrogels in the form of fibers are known. However, although such existing natural or synthetic fibers have a certain extent of water-swellability, some have an extremely low water-swellability and others are water-soluble, and in any case they have been far from the category of water-swellable fibers which can absorb and retain an amount of water several to several hundred times their own weight, and moreover are water-insoluble. In Japanese Laid-Open (Kokai) Patent Application No. 42916/1977, there is disclosed a highly swellable fiber-shaped structure composed of arcylic fibers into which specific cross-linkages and a large amount of salt-form carboxyl groups have been introduced. However, since this fiber-shaped structure contains an extremely large amount of salt-form carboxyl groups introduced thereinto and is hydrogelled throughout the outer and inner layers of the fibers, it is provided, on the one hand, with a high degree of water-swellability indeed, but on the other hand, it is so brittle that its physical properties are far from the conception of fibers. That is to say, it is the present situation that there are no water-swellable fibers having satisfactory properties, and thus the giving of a high degree of water-swellability and the retention of fiber physical properties have been a contradictory problem.